Answer:
Binary file.
Explanation:
In this case, the network security analyst is using the Follow TCP Stream feature in Wireshark to rebuild the TCP transaction. However, the transaction data seems indecipherable. The explanation for this is that the TCP transaction file is a Binary File and because of this the network secuirty analyst is unable to decipher the transaction file.
The solution for this is to convert the binary file into text file which is human readable.